The magnetoresistance effect is generally a physical phenomenon wherein a magnetic field is applied to a magnetic material to vary the electrical resistance. The GMR device that utilizes the giant magnetoresistance (GMR) effect discovered in metallic multilayer film structures comprised of laminations of ferromagnetic metal layers/nonmagnetic metal layers/ferromagnetic metal layers is already utilized in magnetic read-out heads. In recent years, the application of magnetic random access memory (MRAM) devices as nonvolatile memories is under study. However, due to the tremendous advances made in magnetic recording density, utilizing magnetoresistance devices in next generation magnetic heads and large capacity MRAM requires making the magnetoresistance ratio (MR ratio) even larger, and making further increases in the sensitivity response to external magnetic fields.
In recent years, magnetoresistance devices (tunneling magneto-resistance devices: TMR devices) that utilize the tunnel current flowing between both ferromagnetic layers have been discovered. In other words, these TMR devices use the tunnel current that flows when a tunnel junction with an insulating layer is inserted between two ferromagnetic layers namely, a ferromagnetic tunnel junction. The magnetoresistance ratio in these ferromagnetic tunnel junction devices exceeds 20 percent (Non-patent Document 1) so there is increasing potential for applications in magnetic heads and magnetoresistance effect memories. The magnetoresistance ratio at room temperature is approximately 40 percent; however an even larger magnetoresistance ratio is needed in order to obtain the output voltage needed for magnetic read-out heads of high density magnetic recording medium.
In MRAM applications using TMR devices, an external magnetic field (an electromagnetic field) is applied to ferromagnetic layer without a fixed magnetic direction (freely magnetized layer) by making an electrical current flow in the external wiring to reverse the magnetic direction of the free layer of magnetization. However, the constantly shrinking size of the memory cell brings about an increase in the magnetic field (magnetic switching field) required for magnetic reversal of the freely magnetized layer that causes an unavoidable increase in the wiring current for writing. Therefore, increasing the capacity of the MRAM causes an unavoidable increase in electrical power consumption. The increased electrical current in the wiring brings the potential problem of the wiring melting. One method for resolving this problem is to cause a magnetic reversal by injecting a spin-polarized spin current (Non-patent Document 2). However, this method for inducing a magnetic reversal by injecting a spin current, increases the electrical current density flowing in the TMR device. This increase in electrical current density might possibly cause the wiring to deteriorate or induce a ferroelectric breakdown in the tunnel insulation (dielectric) due to electro-migration.
In recent years, on the other hand, much attention has been focused on new magnetic material capable of being utilized in magnetoresistive devices. These are diluted magnetic semiconductors such as type III-IV semiconductors typically of GaAs, InAs or type IV semiconductors of germanium, substituted in part with the magnetic atom manganese. In particular, structures of field effect transistors (FET) fabricated using a magnetic semiconductor as the substrate are capable of inducing a magnetic phase transition from a non-magnetic phase to a ferromagnetic phase by applying an external voltage from the gate electrode to the internal portion of the magnetic semiconductor substrate (Non-patent Document 3).
This electric field-induced magnetic phase transition is caused by the fact that the ferromagnetic transition temperature of the magnetic semiconductor is strongly dependent on the carrier (hole) concentration. The above FET device is an innovative device that utilizes this physical effect to cause shifts in the bulk magnetic state (ferromagnetic state/non-magnetic state) even under fixed temperature conditions, by effectively injecting (doping) carriers the semiconductor internally by applying an electrical field externally. Besides diluted magnetic semiconductors, another material known to be capable of this type of electric field-induced magnetic phase transition is manganese oxides having a perovskite crystalline structure. However, the internal magnetic semiconductor substrate of this FET is not comprised of a ferromagnetic/nonmagnetic/ferromagnetic junction structure, and so no improvement in the MR ratio can be expected even in the vicinity of the magnetic phase transition. This FET structure is therefore not suited for use as a magnetoresistive device.
The diluted magnetic semiconductors are predicted to be a half metal (spin polarization of the fermi surface equals 100%) by an elaborate calculation of electronic states. Based on this prediction, the laminated magnetoresistive device with a nonmagnetic semiconductor layer inserted between the magnetic semiconductor layers is expected to have an MR ratio in excess of 100%. Even though, trial fabrication of ferromagnetic junction type magnetoresistive devices using magnetic semiconductor has also begun very recently, a magnetoresistive device having an MR ratio in excess of 40 percent at room temperature has not been achieved.
[Non-patent Document 1]
J. Appl. Phys.79, 4724 (1996)
[Non-patent Document 2]
Appl. Phys. Lett. 78, 3663 (2001)
[Non-patent Document 3]
Nature 408, 944 (2000), Science 295, 651 (2002)
The magnetoresistive device of the related art typically contains a three-layer laminated heterogeneous junction structure comprised of a ferromagnetic layer, nonmagnetic layer, and ferromagnetic layer. However fabricating a satisfactory heterogeneous interface/junction and a uniformly thin intermediate layer is extremely difficult. Microscopic irregularities at the crystal boundaries of the heterogeneous junction and the effects of microscopic pin holes formed unintentionally in the nonmagnetic intermediate layer create the problem that the MR ratio of the magnetoresistance device drastically deteriorates from the theoretically expected value. Further, the MR ratio in the magnetoresistive device, varies according to the film thickness of the nonmagnetic intermediate layer so that obtaining a large improvement in the MR ratio and optimizing with the peripheral circuits could only be achieved by experimentally fabricating large numbers of devices with nonmagnetic layers of different film thickness.